1. Field of the Invention
The present invention relates to magnetic heat pumps and refrigerators and more particularly such magnetic devices used at cryogenic temperatures.
2. Discussion of Related Art
A. Cryogenic Refrigeration
The term "cryogenic region" refers to very low temperature, from absolute zero Kelvin (equal to -273 Centigrade or -459.63.degree. Fahrenheit) to about -330.degree. Fahrenheit, although there is not clear-cut ceiling. The "cryogenic fluids" are substances which are liquid at cryogenic temperatures and include nitrogen (-196.degree. C. - B.P., 77, hydrogen (-253.degree. C. - BP, 20.3.degree. K.) and helium (-269.degree. C. - BP, 4.2.degree. K.). Nitrogen is often used as it is chemically inert and relatively inexpensive.
One type of refrigerator used to obtain cryogenic temperature is the "ADL-Collins Helium Liquefier" in which helium gas is processed through a four-stage compressor (four compressors operate from the same motor). At each stage the heat added by compression is removed with water. The helium gas is then cooled by a series of "heat engines" (expansion engine cycle) in which the gas work in moving a piston and flywheel performs to lower its temperature and pressure.
An alternative type of cryogenic refrigerator uses "magnetic cooling" and "adiabatic demagnetization of paramagnetic material". It may be used to obtain extremely low temperatures, for example, below 4.2.degree. K. A paramagnetic material acts like a magnet when in a magnetic field but stops being magnetic when the field is removed. The process is "adiabatic" as the material does not gain or lose energy when being magnetized and demagnetized. When demagnetized the paramagnetic or ferromagnetic material has its electron spins in disorder, i.e., it has a high degree of "entropy" (in thermodynamics the degree of disorder in a system). In the magnetic field the electron spins of the paramagnetic or ferromagnetic material are aligned and energy is released to the host lattice (the entropy becomes less) raising the temperature of the host lattice, which temperature rise is removed in the Hot Heat Exchanger (HHEX). Conversely, when the magnetic field is removed the electron spins reassume their random state taking energy from the host lattice so that the lattice becomes colder, and the now colder paramagnetic material is brought into contact with the Cold Heat Exchanger (CHEX).
Presently available cryogenic refrigerators, in general, are costly and they may be inefficient and large. The particular disadvantages of magnetic cooling, i.e., magnetic heat pumps (MHP) are discussed below. The general disadvantages of cost, size and efficiency have prevented the use of cryogenic refrigeration in many laboratories, hospitals and industrial facilities where cryogenic temperatures could be beneficially employed.
Refrigeration at above-cryogenic temperatures, for example, at ambient temperature, suffers from the same problems, particularly that of efficiency. For example, home refrigerators, although they may have a theoretical efficiency of about 70%, in practice operate at below 40% efficiency. In addition, they use Freon (TM) or other chlorofluorocarbon gases (CF) which are detrimental to the ozone layer. In that type of refrigerator, the CF gas (refrigerant) is compressed raising its temperature, which is then cooled by air passing over the radiator coils ("condenser") of a HHEX o the back of the refrigerator. The cooled gas, which may become a liquid, generally at ambient (room) temperature and high pressure, passes through a "Joule-Thompson" nozzle (an expansion valve) so that its pressure and temperature drops and it is then circulated through the coils of tubing inside the refrigerator (the "evaporator", a type of CHEX) in which it absorbs heat and is then recirculated.
One of the most important uses of cryogenic temperatures is to cool selected materials so they become "superconductive", i.e., their electrical resistance drops to zero at the materials' "critical temperature". For example, certain older superconductive materials may be formed into a wire which is coiled about a magnet to form an electromagnet, such as a solenoid, or may be used in an electronic device. For example, the alloy niobium-tin is an older superconductive material which becomes superconductive in the 18.5.degree. K. range
In the past few years superconductors have been developed which become superconductive at higher temperatures.
B. Magnetic Heat Pumps
Magnetic heat pumps (MHP) have been the subject of extensive study for the last 30 years. In the last few years these developments have resulted in workable and efficient systems, particularly for temperatures below 20.degree. K.
In general, in magnetic heat pumps, a magnetic field is applied and withdrawn relative to a material with paramagnetic or ferromagnetic magnetic propertics. There is a transfer of heat between reservoirs at differing temperatures, i.e. between a hot heat exchange (HHEX) and a cold heat exchange (CHEX), which is cooled. When heat extracted from the low temperature heat exchange (CHEX) the device needs input of work, usually physical motion from a motor, and the device is a refrigerator or a heat pump.
While there are a number of theoretical effects in which a magnetic field can be used in a thermodynamic cycle, only the "magnetocaloric effect" (MCE) has been commercially exploited. In the magnetocaloric effect, a paramagnetic or ferromagnetic material is exposed to a very high magnetic field, for example 3-12 Telsa, as a result, electron spins within the material are forced to align. When that occurs, the entropy of the system decreases (the order in the system is higher). When the process is adiabatic, the temperature of the magnetic material rises, and in non-adiabatic processes, heat is exuded to the environment. Conversely, if the material is already under the influence of a strong magnetic field, and this field is removed adiabatically, the electrons' spins reassume random positions taking heat from the lattice, (less order, thus more entropy) and the material' s temperature decreases. When the magnetic field is removed non-adiabatically, heat is absorbed by the magnetic medium.
It has been speculated that the availability of high temperature superconductors will have a major impact on the potential commercial feasibility of such MCE heat pump systems because they will be used in electromagnets to form high magnetic fields with little or no Joule heating losses. However, MCE heat pumps have an intrinsic shortcoming that cannot be remedied with the potential availability of superconductor based extremely high magnetic fields. In the best MCE materials, (usually gadolinium compounds), the maximum temperature lift achievable between the heat sink and heat source is about 20.degree. K. (magnetic field strength required is about 8 Tesla). This is due to the fact that once most of the electron spins have been aligned, saturation sets in and additional increases in the magnetic field do not improve the process.
In theory classical low temperature superconductors may exhibit an "inverse magnetocaloric effect," in which such a superconductor exposed adiabatically to a quenching magnetic field, cools, and when the quenching magnetic field is removed, the superconductor heats up. The entropy of the superconductive phase is always lower than the entropy of the normal phase at the same temperature (the superconductor being quenched to the normal phase by an externally applied magnetic field which is larger than its critical field at that temperature). The difference in entropy is rooted in the fact that in the superconducting phase, the electrons are paired and thus provide a more ordered system. The free energy difference between the two states is actually the sum of the binding energies associated with the coupling of all the electron pairs in the superconductor.
However, the inverse magnetocaloric effect in low temperature classical superconductors is not a practical means of refrigeration for a number of reasons. First, due to the very small binding energy of classical superconductors (at most 3 millielectron volts per pair), the quantity of heat extractable per cycle from the classical superconductors is small. Second, the thermal conductivity of the classical superconductor, in their superconducting phase, is very low, making it difficult to build efficient heat transfer machines. Third, the highest maximum critical temperature possible with the classical superconductors is below 23.degree. K. Furthermore, in an inverse magnetocaloric effect using classic low temperature superconductors, relatively large fields are required to obtain the desired effect, since at least the lower critical field Hcl must be exceeded which is quite high in classical superconductors.
C. Superconductors
Until recently, it was believed that superconductivity above 23.degree. K. was not possible. This belief was rooted in the theoretical work now named the BCS theory (Bardeen, Cooper and Schrieffer) which predicted such a limit. In the context of this invention the term "high temperature superconductor" (called "high Tc material") means any superconductor whose critical temperature is above the limit set by the BCS theory, namely with a critical temperature in excess of 23.degree. K.
In the early 1970's a number of theoretical proposals were presented, suggesting that the critical temperature for superconductivity could be increased above the BCS limit. (V. L. Ginzburg, Usp. Fiz. Nauk. 101, 185 (1970)) (D. Allender, J. Bray, J. Bardeen, Phys. Rev. B8, 4433 (1973)) A significant experimental breakthrough in high temperature superconductivity was provided in November 1986 by Bednorz and Muller (of IBM Zurich) when they published a tentative disclosure of high temperature superconductivity (Georg Bednorz and Alex Muller, Z. Phys. B64, 189 (1986)).
Early confirmation came from Japan in November 1987 where the Meissner effect was reported for high-Tc material. In a second paper for La(2-x)Ba(x)CuO(4-y), a critical temperature above 30.degree. K. was reported (H. Takagi, S. Uchida, K. Kitazawa, S. Tanaka, Jpn. J. Appl. Phys. 26, L123 (1987)).
In the United States, confirmation of a 93.degree. K. superconductivity transition temperature was reported by Chu for yttrium-barium-copper oxide ceramic. (M. K. WU, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, Mar. 2, 1987, p. 908.)
This material was dubbed the 123 or 1-2-3 compound and serves as a model for advanced research in the field.
During 1987 and 1988, a number of families of high temperature superconductors were discovered with confirmed critical temperatures all the way to 162.degree. K. These materials are usually ceramic oxides containing copper, usually in the trivalent state, an alkaline metal (Ca, Sr or Ba) and a rare earth including yttrium.
In later developments the rare earths have been partially, and sometimes completely replaced with bismuth, or combinations of bismuth and lead to yield critical temperatures between 90.degree. K. and 160.degree. K. Similar substitutions with thallium yield another family of superconductors with a similar range of critical temperatures. There are even some scattered unconfirmed reports of superconductivity above 162 .degree. K. , for instance by R. G. Kulkarui reporting superconductivity in the vicinity of 200.degree. K. , in a mixed spinel oxide having the approximate composition Ca(0.5)Zn(0.5)Fe(2)O(4). Also Ogushi has reported room temperature superconductivity in yet ill defined niobium, strontium, lanthanum oxides. Late in 1989, scientists at Wayne State University (J. T. Chen at al.) have reported superconductivity in a compound related to 1-2-3 at temperatures as high as 240.degree. K.